Some pessimists "say there is no prospect of reform in the European Union, you simply have to leave. I think they're wrong too. I think it is possible to change and reform this organisation and change and reform Britain's relationship with it. But as I said, it is in Britain's interests to remain a country that is uniquely well-connected around the world and that is why I have set out a plan for how we reform theEuropean Union and then we give Britain a choice about whether to stay in thatEuropean Union or not," Cameron told an investment conference.

Cameron came under renewed pressure from EU sceptics this week when former finance minister Nigel Lawson said the prime minister's plan to renegotiate Britain's commitments to the EU before a planned membership referendum in 2017 were doomed to fail and the country should leave the bloc.

Cameron also used his speech on Thursday to underscore his determination to keep on narrowing Britain's budget deficit at a "sensible and measured pace" and to help push for new trade deals between the EU and the United States andCanada.

He also said he would continue to defend Britain's financial services industry against some European measures such as a planned financial transaction tax which has been agreed by most countries in the euro zone and would affect the City of London.

"We shouldn't spend our time in politics endlessly bashing banks and financial institutions. If you want an economy to recover, if you want an economy to grow, you have to play to your strengths," Cameron said.

Cameron came to power in a coalition government in 2010 with a plea to his party to "stop banging on about Europe", an issue that has divided the Conservatives for decades and helped bring down two of his predecessors, Margaret Thatcher andJohn Major.